(1) Field of Invention
The present invention pertains to an inkjet printhead capable of jetting ink droplets onto recording medium or media (recording paper) to form image(s) and to an inkjet image forming apparatus equipped with such inkjet printhead(s). In particular, the present invention relates to a strategy for achieving improved ink droplet jetting performance through stabilization of jetted ink droplet velocity.
(2) Conventional Art
Inkjet-type image forming apparatuses (hereinafter referred to as “inkjet printers”) typically carry out image formation by jetting ink droplets onto the surface of recording paper fed therethrough. That is, processing is carried out to create multivalued representations, binary or higher in number of values, of images to be formed, and prescribed dots are formed on recording paper by carrying out controlled jetting of ink droplets from respective nozzles of an inkjet printhead based on a dot ON/OFF signal obtained as a result of such processing.
Furthermore, various types of mechanisms have been proposed for carrying out such jetting of ink droplets. As disclosed for example at Japanese Patent Application Publication Kokai No. S63-247051 (1988), one such mechanism is of a type wherein pressure for jetting of ink droplets is obtained through employment of a piezoelectric member. More specifically, as shown in FIG. 15, plurality of cavities 102, 102, . . . are formed in base plate 101 comprising ceramic or other such piezoelectric material, partitions 103, 103, . . . partitioning respective cavities 102, 102, . . . being polarized in the direction of the depth of ink chambers 104, which correspond to the spaces at the interior of cavities 102, and drive electrodes 105 being formed at prescribed regions (e.g., the upper halves) of these partitions 103. Furthermore, cover plate 106 is attached over base plate 101 so as to close off the tops of these cavities 102. Note that the foregoing respective cavities 102, 102, . . . are formed by cutting using a diamond blade or the like. Furthermore, drive electrodes 105 are formed by sputtering or the like.
In addition, by separately applying pulsed voltages corresponding to image signal(s) to respective drive electrodes 105, 105, . . . , differences in electric potential are created between respective drive electrodes 105, 105, . . . , causing electric fields perpendicular to the foregoing direction of polarization to be produced. As a result of the piezoelectric shear strain effect which is produced at this time, respective partitions 103, 103, . . . undergo shear deformation. This deformation produces a pressure wave within each such ink chamber d, this pressure being responsible for ink droplet jetting action.
This shear deformation action of partitions 103, 103, . . . is typically such that after applying jetting voltage pulse(s) to prescribed drive electrode(s) 105 so as to actuate partition(s) 103 in a direction such as will cause expansion of ink chamber(s) 104, non-jetting voltage pulse(s) is or are applied to prescribed drive electrode(s) e so as to actuate partition(s) 103 in a direction such as will cause contraction of ink chamber(s) 104. As a result thereof, a pressure wave is made to operate on the ink within each such ink chamber 104 so as to cause jetting of ink droplet(s) from this ink chamber 104 by way of ink nozzle(s), not shown.
Also commonly known in the context of such inkjet printers are multidrop-type image forming operations wherein density gradations are achieved by varying the number of ink droplets delivered per dot on recording paper without changing the size of the ink droplets jetted from respective nozzles (see for example Japanese Patent Application Publication Kokai No. H11-170521 (1999)). With such image forming operations as well, controlled jetting of ink droplets from ink chambers is carried out by controlling voltages applied to respective drive electrodes such as has been described above.
Next, the relationship between ink temperature and the velocity of the jetted ink droplets is described. FIG. 16 shows the change in ink viscosity η (cp) as a function of ink temperature (° C.). As shown in this FIG. 16, the viscosity η (cp) of ink jetted from an ink nozzle varies widely as a function of ink temperature (° C.). For this reason, the low viscosity η of ink at high temperature causes ink droplets to be jetted from ink nozzle(s) at high velocity, and conversely, the high viscosity η of ink at low temperature causes ink droplets to be jetted from ink nozzle(s) at low velocity. The velocity of jetted ink droplets thus varies widely as a function of ink temperature, and such variation in velocity may be accompanied by shift in the location at which ink droplets land, creating opportunities for deterioration in image quality. In particular, in a low-temperature worst-case scenario, where ink viscosity η becomes markedly high, it is possible that jetting of ink might stop completely or that inkjet printer jetting performance would be otherwise severely compromised.
One method for solving this problem is to control jetted ink velocity by varying the voltage Vp (V) applied in order to produce the electric field at the piezoelectric member (the aforementioned partition 103) of the printhead in correspondence to changes in printhead temperature (° C.), i.e., ink temperature (° C.), so as to ensure satisfactory inkjet printer jetting performance. That is, as shown in FIG. 16, constant jetted velocity of ink droplets is maintained regardless of ink temperature (° C.), and deterioration of image quality is avoided, by causing the applied voltage Vp (V) to be set higher for lower ink temperatures (° C.).
However, with the aforementioned method in which applied voltage Vp is varied in correspondence to changes in ink temperature so as to achieve constant jetted velocity of ink droplets, the drive circuitry for jetting of ink droplets will require temperature sensors, variable voltage circuits, and so forth. This consequently creates a new problem in the form of the increased burden which is placed on the inkjet printer drive circuitry (first problem).
On the other hand, heat generated by a piezoelectric member contributes to increase in temperature of the piezoelectric member as well as surrounding circuitry, affecting the characteristics and longevity of the piezoelectric member itself as well as the surrounding circuitry. For this reason, stratagems such as those by which heat generated by piezoelectric members is dissipated through structural means have conventionally been devised. Disclosed at Japanese Patent Application Publication Kokai No. H9-48113 (1997) is a structure for preventing reduction in jetting performance due to changes in ink temperature, the structure being capable of preventing reduction in jetted ink velocity in low-temperature domains, despite the fact that the voltage which is supplied to the piezoelectric member is held constant, as a result of employment of a piezoelectric member having characteristics exhibiting a small rate of change of the electromechanical coupling coefficient with respect to temperature, the rate of change of the electromechanical coupling coefficient with respect to temperature being not more than 3,000 ppm/C.° at least in temperature domains of 20° C. or lower.
However, in conventional inkjet image forming apparatuses, suppression of the amount of heat generated by the piezoelectric member itself has not been carried out. That is, despite the fact that the amount of heat produced by the piezoelectric member itself when the temperature of the piezoelectric member rises may have been lowered as a consequence of the variation of applied voltage Vp in correspondence to change in ink temperature which has been carried out in conventional constitutions, such conventional constitutions have been devoid of any technology which would focus on reducing the amount of heat generated by this piezoelectric member and which would actively utilize same. Furthermore, due to the fact that it has only actually been possible to carry out control of applied voltage at intervals occurring at some fixed period and due to the fact that correction of jetted ink velocity has likewise only actually been achievable at some fixed period, further improvements in ink jetting performance have been difficult to accomplish. Indeed, at Japanese Patent Application Publication Kokai No. H9-48113 (1997), whereas attention is given to the rate of change of the electromechanical coupling coefficient with respect to temperature and there is improvement of ink jetting performance within low-temperature domains, suppression of the amount of heat generated by the piezoelectric member is not carried out and further improvement of ink jetting performance in domains other than the low-temperature domain would be difficult (second problem).